This invention relates to the binding of chlorine gas, and in particular the discovery of a new medium and method for binding chlorine gas.
According to The Chlorine Institute, Inc., the United States produced 13.5 million short tons of chlorine in 1998. A known process for commercial production of chlorine comprises electrolyzing a water solution of sodium chloride. During the production process, approximately 95% to 96% of the chlorine is liquefied and recovered in condensers. The remaining 4% to 5% of the chlorine is not liquefied, but rather entrains with non-condensable gases. This so-called xe2x80x9csniffxe2x80x9d gas, or offgas, may be neutralized with caustic, or subjected to further processing for recovery of meaningful amounts of the remaining chlorine. Recovery may be accomplished by using commercially available, specialty absorbents and systems that include crystalline molecular sieves (including zeolite molecular sieves), activated carbons, activated clays, silica gels, activated aluminas, and the like. Recovery results may however vary considerably.
While efficient industrial recoveries may ultimately release to atmosphere vent gas streams that have about 10 parts per million chlorine, by volume, less efficient recoveries may release vent gas streams that contain up to about 100 parts per million chlorine, by volume. When a vent gas stream is released to atmosphere, the amount of entrained chlorine gas is a matter of importance.
In a non-liquid phase, chlorine is a greenish yellow gas. It is a strong oxidizer that is caustic and toxic. Its odor is detectable by human smell at about 0.3 parts per million and above, and its irritation threshold is approximately 0.5 parts per million. The National Institute for Occupational Safety and Health considers gaseous chlorine at 10 parts per million and above immediately dangerous to life and health.
Chlorine is used in the manufacture of a wide range of products. Approximately one-third of manufactured chlorine is used in vinyl chloride monomer for the manufacture of polyvinyl chloride (PVC). About one-tenth is used for bleaching by the pulp and paper industry, although this use has been declining as alternative, environmentally benign bleaching methods have been developed. The water treatment industry uses about 5% of the chlorine produced. Other uses for chlorine include the manufacture of a wide range of chemicals including chloroethane, titanium dioxide, propylene oxide, ethylene dichloride, epichlorohydrin, and chloromethanes. It is believed that industry produces approximately 11,000 different commercial chemicals from elemental chlorine.
Despite safeguards that may be in place for the commercial production and use of chlorine, releases of chlorine gas can occur, such as the example of non-recoverable chlorine gas that may be present in vent gas streams that are emitted to the outdoors. Failure to limit the amount of chlorine in a vent gas to less than a harmful amount may have hazardous consequences. Accordingly, it is known to use air column filters based on activated carbon to bind the chlorine in vent gas and thereby preventing its release into the environment. Although some may consider activated carbon relatively costly and relatively poor in absorption efficiency, it continues to enjoy significant commercial use, largely it is believed because no better alternatives or replacements have yet emerged. Accordingly, it is believed that an alternative to activated carbon for binding chlorine gas would be useful, and would enjoy significant commercial acceptance especially if it can bind chlorine gas with greater efficiency and better cost effectiveness than activated carbon.
While various commercial products and methods exist for removing halogens, including chlorine, from tap water and waste water, activated carbon enjoys substantial commercial use as binding medium for binding chlorine in water. An alternative to activated carbon for dechlorinating water appears in a publication of the former Soviet Union, specifically in Inventor""s Certificate No. 810612 published Mar. 7, 1981. An English translation of that document describes a process that uses sphagnum, a plant-based fiber, to dechlorinate water and a process for regenerating the chlorine-saturated sphagnum using a sulfite alkaline solution containing sodium sulfate and caustic soda (sodium hydroxide).
According to Water Treatment by Tillman (Ann Arbor Press, Inc., 1996), chlorine dissolves in water according to the reaction:
Cl2+H2OHClO+HCl
which is accompanied by the secondary reaction:
HClOClOxe2x88x92+H+
Tillman continues with the observation that the direction of these equilibrium reactions depends on the pH value of the water. For a pH value below 2, the chlorine is present is molecular form. As the pH value is increased to 5, the molecular chlorine disappears entirely to recur as hypochlorous acid. At a pH value of 10, the chlorine is combined in the form of hypochlorite ions. Between pH 5 and pH 10, which is usually the case with water subject to chlorination, a mixture of hypochlorous acid and hypochlorite ions forms, their relative proportions varying according to the pH of the solution.
Authorities in physical chemistry have discussed how the adsorption of gases on solids differs from adsorption from solution. In this regard, the reader is referred to Chapter 13, xe2x80x9cSurface Chemistryxe2x80x9d found in Walter J. Moore, Physical Chemistry, 2nd Edition, Prentice-Hall, Inc. 1955, and to Arthur W. Adamson, Physical Chemistry of Surfaces, 3rd Edition, John Wiley and Sons, 1976. The binding of ions from solution by a solid in the solution seems to involve electrical phenomena at the interface, and this would be different from a gas-solid interface where a gas is quite unlikely to be ionized. In other words, the mere fact that the Russian document describes an ability of sphagnum to dechlorinate water, where the chlorine would be understood not to be in the molecular state, is not seen to suggest, much less assure, that sphagnum can bind gaseous chlorine where the element is in the molecular state.
Moreover, sphagnum possesses a known ability for binding cations and heavy metals, as discussed for example in U.S. Pat. No. 4,861,481. The ability of sphagnum to bind cations offers a logical explanation for a reason why sphagnum is used as a potting soil ingredient. In addition to its ability to absorb water, sphagnum""s ability to bind cations would be beneficial in binding plant fertilizers, many of which are cations. However, this does not seem to offer an explanation for the apparent ability of chlorine to bind the anions which would be present in chlorinated water. Hence, the inventor believes that a different mechanism is at work.
The present invention involves the discovery that sphagnum, sphagnum moss, and sphagnum peat can bind chlorine gas where the chlorine is in its elemental state. Moreover, it has been discovered that these organic substances can copiously bind chlorine gas with greater efficiency than the activated carbon that has continued to enjoy substantial, and what is believed to be essentially dominant commercial use to the exclusion of other binding media.
Accordingly, one generic aspect of the present invention relates to a method of binding chlorine gas comprising a) providing organic matter selected from the group consisting of sphagnum, sphagnum moss, and sphagnum peat moss, and b) contacting the organic matter with chlorine gas.
Another generic aspect of the present invention relates to a medium for binding chlorine gas comprising organic matter selected from the group consisting of sphagnum, sphagnum moss, and sphagnum peat moss.
What now follows is a detailed description that will serve to disclose principles of the invention in accordance with a best mode contemplated at this time for carrying out the invention.
This invention uses a plant-based fiber, specifically sphagnum, sphagnum moss, or sphagnum peat moss, to bind elemental chlorine in its gaseous form. As used herein the term sphagnum includes all species of living plants within the genus sphagnum whether naturally growing or cultivated. Sphagnum moss includes sphagnum that has been harvested, regardless of whether it has been subjected to further processing such as milling. Sphagnum peat moss includes partially decomposed sphagnum and/or partially decomposed sphagnum moss.
The ability of sphagnum, sphagnum moss, and sphagnum peat moss to bind chlorine gas has been demonstrated by experiment. In an environment of chlorine gas, it was found that sphagnum moss can bind up to 10% of its dry weight in chlorine. This contrasts with activated carbon, the industry standard, which under identical experimental conditions was found to bind up to 1.6% of its dry weight in chlorine. Thus sphagnum moss has a six-fold greater ability to bind chlorine gas than does activated carbon. Moreover, sphagnum moss has the ability to bind chlorine from chlorine gas regardless of fiber particle size or moisture content.